Rotating joint



Jan. 18, 1955 RAGAN 2,700,137

I ROTATING JOINT Filed March 5, 1946 INVENTOR GEORGE L. RAGAN BY W ATTORNEY United States Patent Q P ROTATING JOINT 1 George L. Ragan Cambridge, Mass, assignor,=. by mesne assignments,.-tothe-United-Statesof America as represented by the Secretary of-therNavy.

AppIiCationMarch JS,1946, Serial No;.6 52,'192' 6 Claims; (Cl. 333-597) This invention pertains; to coaxial cables, as are used te -conduct" electrical-energyy'and more "particularly to section.10 andthe free endof cylindrical -sleeve 16, the,

extent of said gapvaryingin responseto relative longi-v tudinal' translation between conducting sections 10 and 12.

In operation; energy travelling ontheouter surface of conducting sectionlll reaches gap,26'.f Because -the length L1 isrnade 'equal to: the half Wavelength ,of energy; for which the cable is intended; such 'energywill'see across gap 26a 'short-circuited resonant line a half-wave :in;

.length. "Iliis"is"equivalent"t0'.a short circuit .directly across:.=gap*26;f-and-"so the energyis transmitted across the gap-with little or no loss.

If the energy has-a frequency suchuthatthethalfwave-; length differs slightly from the lengthLganalysis requires by lengthsLz andls.

v.consideration of sections defined L2 willapproximate, butwill not equal, a .quarter wavelength." The-characteristic impedanceofthe coaxial line of lengthLz is relatively'hi'gh, because of thehigh ratio of dimensions c/d:

tinuityu Difficulties haveexisted, however, characteristic of the solutions used. Contacting-type couplings worerapidly and consequently soon became unserviceab le. Capacity-type'couplingscommon in the prior artrequire close tolerancesin'alignmentand spacing, and these tolerances are not easily established or maintained. Thisin; vention'presents-a solutionto-the'problem of installing a rotating jointin---the'inner-conductor.only, a solution; which eliminates-much-of-'the-difiiculty resulting from ;.the objections: described above.

his an object of "this invention to provide a novelrotate; ing joint for-the inner-conductor of a coaxial cables Another object of this invention is to provide a rotating joint for the inner conductor of a coaxial cable, said joint being designed to experience only low currents in those areas subject to mechanical wear.

A further object of this invention is to provide a rotating joint for the inner conductor of a coaxial cable, said joint to preserve effective continuity of the outer surface of aforesaid inner conductor for the mid-band frequency for which aforesaid cable is designed, and to present only low impedance along aforesaid outer surface to frequen cies differing slightly from aforesaid mid-band frequency.

Other objects and advantages of this invention will become apparent from study of the accompanying claims and description taken in connection with the drawing, the single figure of which is a longitudinal section through a joint as presented and described herein.

Referring to the drawing there is shown a section of coaxial cable including a rotating joint. The outer conductor joint features capacity coupling as used in the prior art, incorporating flanges as shown. Of the inner conductor, a section 10 adjoins a similar section 12. It is desired to provide between conducting sections 10 and 12 unlimited relative rotation, slight relative longitudinal translation and, concurrently, the best possible effective electrical continuity. A coaxial cylindrical extension 14 of reduced diameter d is formed on the end of conducting section 12. Extension 14 cooperates with that portion of a cylindrical sleeve 16, having inner diameter c, which surrounds extension 14, thus forming a short length L of high-impedance coaxial line. Sleeve 16 is fixedly secured at one end to conducting section 12, the other end being free,.as shown in the figure and as explained below. A coaxial cylindrical extension 18 of reduced diameter b is formed on the end of conducting section 10. Extension 18 cooperates with that part of sleeve 16 which surrounds it to constitute a short length L3 of low-impedance coaxial line. Lengths L2 and L3 are equal and together constitute the length L1, a halfwavelength for energy of the cables mid-band frequency. A conducting element 20, coaxially mounted in extension 14, protrudes through a bearing 24 mounted in extension 18 and into a coaxial cylindrical recess 22. Relative motion, both rotatable and slidable, is permitted between bearing 24 and conducting element 20. The bearing may be of any suitable type, that in the figure being for example only. It is desirable that some electrical contact be maintained at all times between bearing 24 and conducting element 20. A gap 26 exists between conducting 3Q because .ofathe low ratio 0 Characteristic impedance=Z i q log c/d This combination-of a short-circuited line,- approximately wavelength; anda high char-- in a'high sending-end im-y equal in length to a quarter acteristic. impedance results pedance: in.'plane .a'a-between lengths L2 and L3.

Thelength L3 also will approximate, but will' not:

equal, aquarterwavelength. The characteristic imped-. ance of "thBiCOflXlll dlilG dimensions-c/-b:-

ol gll) This combination of a line, in length approximating a quarter wavelength, terminated (in a plane aa) by a high impedance (as explained above) results in a low sending-end impedance at gap 26.

In other words, even when the frequency departs slightly from that mid-band frequency for which L1 equals a half-wavelength, there still exists at gap 26 a good admittance for passing energy from conductor 10 to conductor 12.

What is claimed is:

l. A rotating joint for an inner conductor of a coaxial cable used to conduct electrical energy of a predetermined wavelength comprising a first conducting section, a coaxial cylindrical extension of reduced diameter formed on one end of said section, a bearing mounted on said extension, a second conducting section, a coaxial cylin drical extension of smaller diameter than the diameter of said first-mentioned extension formed on one end of said second-mentioned conducting section, a conducting element electrically connecting said bearing and said second-mentioned extension, and a cylindrical sleeve coaxially disposed over said extensions mounted in mechanical and electrical contact upon said second-mentioned conducting section providing a clearance gap between one end of said sleeve and said first-mentioned electrical conductor and providing a half-wavelength line to minimize impedance across said gap, the length of said extensions each being substantially equal to a quarter-wavelength of the electrical energy being conducted by said cable to provide a minimum impedance at said gap and to provide a minimum current fiow at said bearing.

2. A rotating joint for the inner conductor of a coaxial cable having first and second substantially abutting axially aligned, inner conductors, an electrically conducted bearing mounted at the end of said first conductor, a conducting element mounted coaxially at the end of said second conductor and protruding through and electrically contacting said bearing, and a cylindrical conducting sleeve of a length substantially equal to onehalf wave length at the frequency of operation secured at one end to said second inner conductor and enclosing said bearing and a quarter wave length portion of said first conductor, the abutting ends of said first and second conductors being of reduced diameter for approximately one quarter wave length, the diameter of said first inner conductor at its abutting end being greater than the diameter of said second inner conductor at its abutting end, and forming with said sleeve one quarter Patented-Jan; 18," 1955 oflength1L3 is relatively 'low' wave length transmission lines of low and high impedance, respectively.

3. A rotating joint for the inner conductor of a coaxial cable having first and second substantially abutting inner conductors, said inner conductors each being of reduced diameter along a length substantially equal to a quarter wave length at the frequency of operation from its respective abutting end, the diameter of the quarter wave length portion of said first conductor being greater than the diameter of the corresponding portion of said second conductor, rotary electrical contact means between the abutting ends of said conductors, and a cylindrical conducting sleeve of a length substantially equal to onehalf wave length secured at one end to said second conductor and enclosing said contact means and forming a clearance gap with said first conductor at the end of said portion of reduced diameter, the reduced diameter portions of said first and second conductors forming with said sleeve quarter wave length transmission lines of low and high impedance, respectively.

4. In a coaxial conductor transmission line including first and second coaxial lines disposed in axial alignment, a rotating joint for the inner conductors of said line comprising, integral extensions of reduced diameters formed on the abutting ends of the inner conductors of said line, the diameter of the extension of the first inner conductor being greater than the diameter of the extension of the second inner conductor, rotary contact means between said first and second inner conductors for mechanically and electrically connecting said conductors, and a conducting sleeve secured at one end to the second inner conductor and extending over said extensions, said sleeve being of a length to provide a clearance gap between the other end of said sleeve and said first inner conductor and forming transmission lines of low and high impedance with the larger and smaller extensions respectively.

5. In a coaxial conductor transmission line system including first and second coaxial lines disposed in axial alignment, a pair of non-contacting and cooperating sleeve chokes mounted on the outer conductor of said lines providing for the transfer of radio frequency electromagnetic energy between the outer conductors of said lines, and means providing for relative rotation of said coaxial lines and for maintaining said coaxial lines in axial alignment comprising, axial extensions of unequal reduced diameter formed on the inner conductors of said lines, the larger of said extensions having a recess formed therein to receive the smaller extension in contacting relation for providing rotary mechanical and electrical contact between said inner conductors, and a conducting sleeve having a diameter substantially equal to the unreduced diameter of said inner conductors and secured at one end to the inner conductor having the smaller extension and having a length equal to a half wave length at the frequency of operation for substantially enclosing both of said extensions and forming with the larger and smaller extensions transmission lines of low and high impedance, respectively.

6. Apparatus in accordance with claim 5 wherein each of said extensions has a length substantially equal to a quarter wave length at the frequency of operation of said system.

References Cited in the file of this patent UNITED STATES PATENTS 2,401,344 Espley June 4, 1946 2,407,318 Mieher Sept. 10, 1946 2,412,867 Briggs et a1 Dec. 17, 1946 2,422,961 Hallstrand June 24, 1947 2,434,509 Okress Jan. 13, 1948 2,451,876 Salisbury Oct. 19, 1948 

